The present invention relates to a brightness control system for an automotive vehicle electronic meter for displaying various measured information by the use of a light emitting display of a fluorescent indicator tube and the like.
There is a growing tendency for electronic meters to be employed to display various measured information in an automotive vehicle, such as residual fuel amount, water temperature, oil pressure, and battery voltage, by using a light emitting display, such as a flourescent indicator tube or a light emitting diode.
Unless the brightness of the light emitting display used in such an electronic meter is properly controlled in response to the ambient light lever, the visibility of the display is spoiled by the reduction of the contrast ratio caused by excessive low intensity or the glare caused by excessive high intensity.
Then, it is conceivable to utilize an illumination controller provided for dimming an illumination lamp of an ordinary meter (such as a speedometer or a tachometer) during the nighttime as a control of the brightness of the light emitting display of the electronic meter in accordance with a variable duty factor pulse signal generated by the illumination controller. The illumination controller is so designed that the duty factor of its output pulse signal varies from 100% to 0%, thereby controlling the intensity of the illumination lamp from zero up to the maximum value.
More specifically, the output terminal of the illumination controller is connected to the ground side terminal of the illumination lamps, therefore, when the output pulse signal of the illumination controller is at its high level, namely, voltage of +12 V, the illumination lamps are turned off since they are not supplied any voltage across their terminals, and when the output pulse signal is at its low level, namely OV, then the illumination lamps are supplied the driving voltage of 12 V and are turned on. Thus, the illumination lamps are supplied with a periodically interrupted driving current according to the output pulse signal of the illumination controller. The brightness the lamps depends upon the variable duty factor of the output pulse signal.
Accordingly, if the brightness of the electronic meter is controlled night and day in accordance with the pulse signal of the illumination controller alone, it may accidentally happen that during the daytime the illumination brightness will be reduced to the lowest level, namely, the zero level, and that, the display will become completely invisible unlike the case of conventional meters of the needle type.
If a driving current circuit of a light emitting display of an electronic meter is connected merely to an illumination controller of the conventional design, the control of the brightness of an electronic meter during the daytime and the control of both of the illumination lamp of an ordinary meter and the electronic meter during the nighttime becomes impossible.